The present invention relates generally to communication systems and, in particular, to a communication system in which the system infrastructure at least occasionally provides power to system subscriber devices.
Communication systems are known to include a system infrastructure and a plurality of subscriber devices. In most communication systems, the subscriber devices (e.g., cellular telephones or wireline modems) include their own DC power source (e.g., a battery or an AC-to-DC transformer which is plugged into an AC wall outlet). However, in standard telephone systems, the system infrastructure provides the minimal DC power necessary to power the subscriber devices (with the exception of cordless telephones which include their own AC-to-DC transformers). The infrastructure provides the power to the subscriber devices in standard telephone systems to ensure that, in the event of a power outage at a subscriber device location, the subscriber device user will still have telephone service, especially with respect to so-called xe2x80x9clifeline servicesxe2x80x9d, such as xe2x80x9c911xe2x80x9d and others.
Telephony service has traditionally been delivered to consumers by regional telephone companies via circuit switched technologies. However, cable (community access television (CATV)) operators are beginning to offer telephony services over their cable systems, using standardized modems and a packetized Internet Protocol (IP) technology generally known as xe2x80x9cvoice-over-IP (VoIP)xe2x80x9d. An exemplary prior art two-way cable system is illustrated in block diagram form in FIG. 1.
The prior art cable system includes headend equipment 101, a hybrid fiber coaxial (HFC) cable plant 103, a plurality of cable modems 105, 106 (two shown), and a corresponding plurality of subscriber communication devices 107, 108 (two shown) coupled to the cable modems 105-106 via corresponding communication links 116, 117. The headend equipment 101 includes processors, routers, switches, a broadband downstream transmitter, upstream receivers, splitters, combiners, subscriber databases, network management stations, dynamic host configuration protocol (DHCP) and trivial file transfer protocol (TFTP) servers, call agents, media gateways, and billing systems. The HFC cable plant 103 includes fiber optic cables, coaxial cables, fiber/coax nodes, amplifiers, filters, and taps which support transmissions from the headend equipment 101 to the cable modems 105, 106 over a shared downstream channel 110 and transmissions from the cable modems 105, 106 to the headend equipment 101 over a shared upstream channel 112.
Each channel 110, 112 utilizes a respective transmission protocol to communicate information over the channel 110, 112. Typically, the modulation used to convey information over the downstream channel 110 (e.g., 64-ary quadrature amplitude modulation (QAM)) is of a higher order than the modulation used to convey information over the upstream channel 112 (e.g., differential quaternary phase shift keying (DQPSK) or 16-ary QAM), resulting in higher speed downstream transmissions than upstream transmissions. Cable systems in which upstream transmission speeds are less than downstream transmission speeds are typically referred to as xe2x80x9casymmetricxe2x80x9d systems. Cable systems in which upstream transmission speeds are substantially equivalent to downstream transmission speeds are typically referred to as xe2x80x9csymmetricxe2x80x9d systems.
In addition to the particular type of modulation used on each channel 110, 112, the shared nature of each channel 110, 112 introduces other protocol requirements. For example, since the downstream channel 110 is shared, the downstream protocol includes addressing information and each cable modem 105, 106 monitors the downstream channel 110 for information packets addressed to it. Only information packets addressed to a particular cable modem 105, 106 (or the attached communication devices 107, 108) or addressed to all cable modems 105, 106 (or the attached communication devices 107, 108) (e.g., broadcast messages) are processed by the cable modem 105, 106 and forwarded to the associated subscriber communication device 107, 108 as appropriate (e.g., telephone, personal computer, or other terminating device). Since the upstream channel 112 is shared, an upstream channel access protocol is used to reduce the likelihood of collisions of communicated information emanating from the cable modems 105, 106. A number of multiple access protocols exist to define upstream channel access, including well-known protocols such as ALOHA, slotted-ALOHA, code division multiple access (CDMA), time division multiple access (TDMA), TDMA-with collision detect, and carrier sense multiple access (CSMA).
Most two-way cable systems abide by and use the upstream and downstream channel protocols defined in the recently-published Data-Over-Cable System Interface Specification (DOCSIS) Version 1.0, which specification is incorporated by this reference as if fully set forth herein. The upstream protocol defined by the DOCSIS standard is a TDMA approach in which timing is controlled by the headend equipment 101 (referred to as the xe2x80x9ccable modem termination stationxe2x80x9d (CMTS) in the DOCSIS standard) and communicated to the cable modems 105, 106 via time stamp synchronization messages transmitted over the downstream channel 110. Thus, in order for upstream communication to occur in an orderly, high quality manner, a time reference in each cable modem 105, 106 must be substantially synchronized with a similar reference in the headend equipment 101 before the modem 105, 106 begins transmitting information provided by the subscriber communication device 107, 108; otherwise, a transmission from one modem 105 may collide with a transmission from another modem 106.
The headend equipment 101 is typically coupled via an appropriate communication link 119, such as a fiber distributed data interface (FDDI) link or a 100 baseT Ethernet link, to an external network 114, such as the public switched telephone network (PSTN) or a wide area packetized network, such as the Internet. Thus, the two-way cable system provides communication connectivity between the subscriber communication devices 107, 108 and other similar devices, Internet servers, computer networks, and so forth via the external network 114.
To support the aforementioned xe2x80x9clifeline servicesxe2x80x9d, cable system operators must provide power (at least temporarily during local power outages) to the cable modems 105, 106 and their attached subscriber communication devices 107, 108 from the headend equipment 101 via the cable plant 103. However, present-day cable modems 105, 106 consume considerable amounts of power (on the order of 9-12 watts per modem currently with a reduction to 4-6 watts as new technologies become available, under normal operating conditions), with the modem""s downstream receiving and processing circuitry playing the most significant role in power consumption. Considering that, in a typical two-way cable system, the headend equipment 101 may service thousands of cable modems 105, 106. Supplying power to such modems 105, 106, even temporarily during local power outages, creates an overwhelming burden on the cable system operators, likely resulting in increased operating and subscription costs.
One approach to reducing the amount of power consumed by the cable modems 105, 106 is to reduce the transmission rate of the downstream channel 110 (e.g., through the use of a low order modulation, such as QPSK or frequency shift keying (FSK)) and, thereby, reduce the power required by each modem 105, 106. While some of the power savings is realized as a result of reduced processing power required to handle the lower data rate, the bulk of the power savings is realized by the requirement that higher modulation schemes require higher-performing lower-noise RF components, which generally require more power. However, such a change in downstream channel modulation would undesirably, and in many cases unacceptably, reduce the maximum downstream channel bit rate.
Therefore, a need exists for a communication system and method of operation that reduces the average power consumed by a cable modem without negatively impacting downstream channel transmission rates, thereby reducing headend equipment power sourcing requirements to facilitate telephony services and maintaining current data-over-cable downstream throughput rates.